An important part of the manufacturing process is coordinate measurement. It is used to map a test piece's shape and to coordinate jig tools. For some manufacturing applications, such as automobile and airplane manufacturing, coordinate measurement with an absolute RMS accuracy of 0.001 inch over a range of 0 to 40 feet would be highly desirable. Such accuracy has been difficult to obtain with currently available measurement techniques. Typically, the measurements are done mechanically, by using calipers or other mechanical gauges, or optically, by using geometric optics or laser radar.
Geometric optical techniques involve some form of triangulation, or determination of distance by comparing angular measurements from different points of view. Triangulation with theodolites may be computer controlled for speed and accuracy. At least two theodolite instruments are required, and setup and operation may be cumbersome and slow. Photogrammetry is another geometric technique involving computer analysis of photographs of the test piece taken with a special high resolution camera from three or more points of view. Photogrammetry can be much faster to set up and yield higher accuracy than triangulation with theodolites, but time-consuming development and analysis of the photographs is required.
Laser radar refers generally to "time-of-flight" sensors that determine distance by the propagation time for laser light. They have an advantage with respect to geometric techniques of coordinate measurement in that each measurement involves only one line of sight and the data acquisition does not involve photographic film or other materials that must be processed and analyzed, delaying results for long periods of time. The following articles discuss various examples of laser radar techniques for distance measurement.
An article entitled "Laser Radar Range Imaging Sensor for Commercial Applications" by K. G. Wesolowicz and Robert E. Sampson, Proceedings of SPIE, Vol. 783, p. 152 (1987), describes an imaging laser radar system employing a single frequency intensity modulation of a GaAlAs laser diode. The target range L is obtained from the following equation: ##EQU1## where .phi.=measured phase delay due to time of flight;
c=speed of light; and PA1 .upsilon.=modulation frequency.
Since the phase delay has an implicit 2.pi. ambiguity, the range measurement has a corresponding ambiguity interval L.sub.a given by ##EQU2## For example, for a modulation frequency .upsilon.of 0.72 GH.sub.z, the interval L.sub.a is 8.2 inches. The 8.2 inch ambiguity interval must be resolved by some other means if this device is to be used for large-scale coordinate measurement on the order of 0 to 40 feet.
The article entitled "Laser-diode Distance Meter in a KERN DKM 3A Theodolite" by A. Greve and W. Harth, Applied Optics, Vol. 23, No. 17, p. 2982 (1984), describes an intensity-modulated laser radar that uses a phase locking technique to measure the relative phase. By varying the modulation frequency, the authors were able at least in principle to remove the ambiguity in the range measurement. However, it appears that this method results in an inadequate degree of accuracy for some applications.
The article entitled "High-Precision Fiber-Optic Position Sensing Using Diode Laser Radar Techniques" by G. Abbas, W. R. Babbitt, M. De la Chapelle, M. Fleshner, J. D. McClure, and E. Vertatschitsch, Proceedings of SPIE, Vol. 1219, p. 468 (1990), describes a linear position sensor with fiber-optic signal distribution. The sensor uses a frequency-chirped, intensity-modulated laser diode with an intensity-modulation bandwidth of 6 GHz. Absolute distance is obtained by determining the beat frequency between the laser modulation and the delayed modulation of the return signal. The beat frequency is found by high-speed digital Fourier transform of the beat signal. This approach has the important advantage that several sensor heads may be connected by fiber optics to the same source and detection module, provided that the possible variations in range to each of the heads do not overlap. However, absolute accuracies of 0.001 inch over 40 feet would require frequency chirps of very high linearity and chirp rates controlled to 2.5 ppm. These specifications may not be practical or cost-effective for this system.
In an article entitled "Utilizing GaAlAs Laser Diodes as a Source for Frequency Modulated Continuous Wave (FMCW) Coherent Laser Radars" by A. Slotwinski, F. Goodwin and D. Simonson, Proceedings of SPIE, Vol. 1043, p. 245 (1989), the authors describe an instrument that uses optical interferometry to generate beat signals between local and time-delayed optical frequencies. The frequency modulation is achieved by thermal tuning of a laser diode cavity length. The thermal tuning is easily effectuated by precisely controlled variation of the laser excitation current and is thus much easier to obtain over large bandwidths than an intensity-modulation chirp. However, this system has a maximum operational range of about 10 feet, which is inadequate for many applications and, like all coherent laser radars, it is sensitive to target surface roughness. Also, high accuracy and reliability can only be obtained with carefully characterized and monitored single-mode laser diodes.
The above exemplary measurement systems do not adequately meet the simultaneous requirements of very high absolute accuracy and large operational range necessary for the coordinate measurement applications which the present invention addresses. Further, these radars are not incorporated into an optical scanning system specifically designed for large-scale coordinate measurement using retro-reflectors or decals on the test piece as target points.
It is thus an object of this invention to meet accuracy and operational range requirements of 0.001 inch accuracy over a range of 0 to 40 feet using a reliable, cost-effective apparatus, that can be conveniently incorporated into a complete coordinate measurement system.